1. Field of the Invention
The invention relates to a turbomachine, and a preferred use of a turbomachiine.
2. Brief Description of the Related Art
In today's deregulated electricity markets it is lucrative to provide reserves to cover peak demand or to bridge resource failures. Under certain circumstances it may be possible to generate more than half of the income with only a few percent of the electrical energy produced daily for the basic and intermediate load.
Peak load resources can be made available in different ways. One way, the provisioning via peak demand of hydroelectric power plants, is for many reasons only possible in the rarest of cases. Technologies such as air cell turbines so far are still considered exotic and, likely will continue to be so into the far future.
For this reason, peak load resources are also made available in thermal plants. This is, for example, accomplished in that power plants are operated in a basic load operation somewhat below full load, for example at 90 to 95% of their nominal power; depending on the type of installation, reserve power then can be mobilized at various speeds. Sometimes gas turbine power plants that can be quickly started up are provided. But both of these approaches have the result that capacities installed at high cost are only incompletely utilized.
In the case of gas turbines--both in solo operation and as a part of combination systems--there is the possibility of operating these machines at times with a more powerful firing device under a type of overload. For this purpose, on the one hand, the hot gas temperature at the turbine inlet can be elevated briefly by several degrees. This method can be implemented without additional equipment expenditure, but is not economical to the extent that it results in a substantial shortening of component life in the hot gas path.
Another possibility is the introduction of water or steam into the hot gas, preferably in the combustor. The reduction in the hot gas temperature achieved by this makes it possible to add more fuel in order to again increase the hot gas temperature to the design point. This method actually means slight losses in economy because of the latent heat of the water steam removed with the waste gas; but, in view of the high prices for peak demand electricity obtainable on the market, such a temporary loss of efficiency can be tolerated.
U.S. Pat. No. 4,928,478 describes a process in which process steam is produced in a waste heat boiler. Part of the steam, which is, for example, not used as a process steam, can be fed to the combustor of a gas turbine. Naturally, the losses in efficiency will be lower if steam from a waste heat boiler is used to increase the power. On the other hand, it also should be noted that the steam generated there can be used substantially more efficiently in the steam turbine of a combination power plant. But such methods nevertheless are very well suited to spontaneously make available additional power.
The introduction of water or steam into the hot gas of a steam turbine on the one hand is limited when the pressure ratio rises above the mass because of additional mass flux. In addition, the introduction of water or steam into the combustor--frequently also directly in the flame zone--interferes with combustion. Especially in connection with premix burners that are operated with a lean mixture, as described, for example, in EP 0 321 809, this may lead to undesired, negative effects on flame stability.
EP 0 795 685 describes the introduction of steam into the working fluid of a gas turbine without inducing negative effects on combustion. Steam generated in a waste heat boiler is used to cool the components subject to a high thermal stress, and after the components have been cooled, the working fluid is added. In addition to an increase in the mass flux through the turbine, an increase in power is also achieved in that the cooling air mass flux necessary for conventional cooling of the components in the combustor and turbine that are subject to a high thermal stress is in this case also directly available to thermal power conversion.
The steam cooling described in EP 0 795 685 does indeed offer a superior cooling effectiveness, but the safety of the cooling leaves room for criticism. A conventionally cooled gas turbine, in which part of the compressed air is branched off and is added through a cooling system to the components subject to high thermal stress, provides inherent safety in that as long as the machine rotates at least a minimum amount of cooling air is available, in particular also after an emergency shut-down. In the case of the steam cooling proposed in EP 0 795 685, in contrast, a number of components may cause immediate break-down of the cooling during operation of the gas turbine, resulting in major damage.
In this regard, GB 2 236 145, for example, proposes to provide an option that would make it possible on a case by case basis to also use compressor air for cooling in a gas turbine with steam-cooled components. EP 0 684 369 finally proposes to connect the cooling system, without any intermediate shut-off elements, with a tapping point on the compressor that has a suitable pressure. This cooling air system is provided with means to introduce steam into the cooling system that displaces air from the cooling system, which results, due to the low cooling air requirement, in a higher power of the gas turbine.
According to EP 0 684 369, the steam supply conduit is provided with a control valve, and a measuring point for the air quantity is integrated in the cooling air system. The steam quantity used for cooling is thus controlled in a defined ratio to the air quantity. Although the solution proposed in EP 0 684 369 is inherently safe with respect to cooling, to the extent that in case of a failure of the steam supply the corresponding amount of compressor air will be immediately available for cooling, the document relates to a steam-cooled gas turbine. In spite of all of the above mentioned advantages, the steam cooling during continuous operation often has the crucial disadvantage that large quantities of processed, highly pure water must be continuously provided in continuous operation.
This means that, with respect to operating technology, air cooling of a gas turbine has significant advantages. There are no high investment costs for a powerful, permanently operating water processing system. This means that in practice permanent water injection is often foregone; steam cooling so far has not been able to become a standard. On the other hand, there is a latent desire to increase the power of a gas turbine by at least temporarily introducing water or steam--or another suitable medium--into the gas turbine cycle or temporarily saving cooling air in order to cover peak demand.